The dynamics of Infectious diseases

Anton Stoltz

Division of Infectious Diseases

Department of Internal medicine

University of Pretoria

GDP and Infectious Diseases

Probability of infection

Ip = (D X S X T X V) / Hδ

• D: Dose (amount) of micro-organism

• S : Site of contact

• T :Time of contact

• V :Receptive host site

• Hδ: Force of combined immunity

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Non-diseased state

Risk and exposure factors

Susceptibility to disease

Biological evidence of

infection

Progression of disease

Death

Infectious Diseases Cycle

http://ocw.jhsph.edu/courses/refugeehealthcare/PDFs/Lecture9.pdf

Risk and exposure factors

Host Defence

Environment

Microorganism

B cells

T cells

Macrophages Bacteria

Innate and adaptive

Local and systemic

Damaged tissues

Shock

Type

Density

Virulence

Human disease or not Susceptibility to

disease

Biological evidence of

infection

No Interaction

Colonization:

Physiological Interaction between host and

micro organism

Efficient host response

INFECTION

Disease of diverse severity

Cure

Tolerance

Chronic non-resolutive

Inflammation

Insufficient

Misplaced

Excessive or uncontrolled

Sub clinical or no disease

Adapted from Discovery medicine, December 2010

Progression of disease

Transmission of infectious Diseases

Reservoir: Man, animal , Inanimate

Susceptible host :

man, animal

Direct

Indirect

Airborne

Vehicle

Vector

Inanimate

Direct transmission

1. Close physical contact e.g. sexually transmitted disease , Skin to skin contact (MRSA):

2. Vertical transmission (Herpes)

Indirect transmission

Vehicle-borne: These infection are transmitted through the agency of water, food, ice, blood, serum, plasma, and other biological products e.g. tissues and organs

Vector –borne: These infection are transmitted by an arthropod or a living invertebrate carrier such as snails

Fomite – borne: Fomites refer to inanimate objects such as handkerchiefs, bed linen, towels, books, spoons, forks, etc., which have been soiled with infective material

Airborne: (Droplet Nuclei )

Large droplets

Small droplets

Droplet nuclei

Particle size

Epidemiological triangle

Agent Biologic: bacteria virus , parasites Chemical: poison, alcohol, smoking Physical: trauma radiation, fire Nutritional: lack, excess

Environment Temperature, humidity, altitude Crowding, housing neighbourhood Water, milk, food

Host Age, sex, race, genetic profile, previous or other diseases,

religion, occupation, marital status, family background, risk behaviour

Host

A person or another living animal including birds and arthropods that affords subsistence and lodgement to an infectious agent under natural conditions

Types of Host

Definitive Host: Hosts in which parasite attains maturity or passes its sexual stage, are primary or definitive hosts.

Obligate Host: This term refers to the only host of infections such as man in measles

Human super-organism (microbiome)

Illustration by Bryan Christie

20 TED talks

Basic reproductive number (R0) For each sick person how many subsequent new people will be infected

Ebola Swine flu HIV Small pox Measles

R0 (R naught)

R0 is less than 1: Each existing infection is causing less than one new infection. In this case, the disease will decline and eventually die out.

R0 is equal to 1: Each existing infection is causing one new infection. The disease will stay alive and stable, but there won’t be an outbreak or epidemic.

R0 is more than 1: Each existing infection is causing more than one new infection. The disease will spread between people and there may be an outbreak or an epidemic.

R0 and epidemics

The 1918 H1N1 flu (R0 of 1.4 – 2.8) killed 50M people

The 2009 H1N1 (R0 of 1.4 – 1.6) killed 284,000

Epidemiological triangle

Agent Biologic: bacteria virus , parasites Chemical: poison, alcohol, smoking Physical: trauma radiation, fire Nutritional: lack, excess

Environment Temperature, humidity, altitude Crowding, housing neighbourhood Water, milk, food

Host Age, sex, race, genetic profile, previous or other diseases,

religion, occupation, marital status, family background, risk behaviour

Examples of disease causing micro-organisms

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Community acquired versus hospital acquired pneumonia

Sources of HAI’s

Patient Flora

Cutaneous

Gastrointestinal

Genitourinary

Respiratory

Contaminated Hospital Environment

Instruments, Fluid, Food

Air, Medications

Invasive Devices

Urinary catheters

Vascular catheters

Endotracheal tubes

Wounds

Endoscopes

Medical Personnel

Colonized, Infected

Transient, Carriers

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This course has been supported by the President’s Emergency Plan for AIDS Relief (PEPFAR) through Centers for Disease Control &Prevention (CDC) under the terms of U2G/PS002710-02

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Penicillin

Erythromycin

Vancomycin

1940 1950 1960 1970 1990 1980

2000

Levofloxacin

Imipenem

Ceftazidime

Linezolid

Tetracycline

Antibiotic resistance

Isolated for millions of years, cave bacteria resist modern antibiotics

Epidemiological triangle

Agent Biologic: bacteria virus , parasites Chemical: poison, alcohol, smoking Physical: trauma radiation, fire Nutritional: lack, excess

Environment Temperature, humidity, altitude Crowding, housing neighbourhood Water, milk, food

Host Age, sex, race, genetic profile, previous or other diseases,

religion, occupation, marital status, family background, risk behaviour

Environment

Environment- Infection rate Airborne particles

r pxq/Q r 1000 000 x 920/∞ m3

r small chance

r pxq/Q r 1600000/100m3

r 160000

C =S(1 − e−Ipt[Q]) Wells-Riley equation

Probability of Infection = s/c

S/C =(1 − e−Iqpt[q/Q]) C = number of new cases S = number of susceptible individuals exposed e = base of natural logarithms I = number of infectors p = pulmonary ventilation rate of susceptible individuals (0.6 m3/h) t = exposure time (hours) Q = absolute room ventilation (m3/h) = [q/Q] q = number of infectious “quanta” produced per hour by infectors Q = volume of disinfected air into which quanta are distributed

Mathematical model of airborne infections

Airborne Organisms

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Airborne spread

Obligate

Tuberculosis

Preferential

Measles

Varicella

Opportunistic

Smallpox

SARS/MERS

Influenza

Roy CJ, Milton DK. Airborne transmission of communicable infection – the elusive pathway. N Engl J Med 2004;350(17):1712-2.

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Non-diseased state

Risk and exposure factors

Susceptibility to disease

Biological evidence of

infection

Progression of disease

Death

Infectious Diseases Cycle

http://ocw.jhsph.edu/courses/refugeehealthcare/PDFs/Lecture9.pdf

https://followtheoutbreak.wordpress.com/2013/10/16/opinion-do-we-need-to-induce-stress-in-the-one-health-paradigm/

Prevention of infection

1. Primary Prevention: Primary prevention is a reduction in the incidence of disease through immunization, sanitation, education, or other means of eliminating pathogenic contamination in the human environment. This is applied in the period of pre-pathogenesis

2. Secondary Prevention: This is applied in the period of pathogenesis and aims at early detection of the disease followed by its treatment Pap smears for the early detection of cervical cancer and surgical intervention if necessary

3. Tertiary Prevention: This is applied in the period of pathogenesis and extends in the period of recovery. It aims at reducing impairments/disabilities and helping patients to utilize the residual capacities maximally

Incubation Period

Prodromal Period Disability

Recovery Latency or

Death

Recurrence of

Symptoms

Sub-Clinical Period Clinical Period

Secondary Prevention Tertiary Prevention

Exposure to an Infectious Agent

Period of Pathogenesis Primary Prevention

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